This invention relates to signal sources for supplying a periodic electrical signal of relatively high spectral purity at a selectable signal frequency. More particularly, this invention relates to selectable frequency signal sources for supplying a signal which exhibits relatively low phase noise.
Signal sources for supplying a periodic electrical signal of relatively high spectral purity at a selectable signal frequency are required in many areas of the electrical and electronic arts. For example, such signal sources are often utilized as the signal generating stages of channelized communications equipment and often serve as test and instrumentation equipment for many purposes including the calibration, testing and maintenance of various electronic equipment.
Virtually all prior art attempts to provide selectable frequency signal sources which exhibit noise and stability characteristics compatible with the most demanding signal source design requirements are either an active oscillator that employs one or more active devices such as transistors in conjunction with tunable resonant circuits that establish positive feedback, or a circuit arrangement commonly known as a frequency synthesizer, which includes one or more active oscillators interconnected with various other circuit stages. As is known in the art, such frequency synthesizers include: (1) arrangements known as direct frequency synthesizers wherein a desired output frequency is obtained directly from the frequency of one or more applied reference signals through the operations of mixing, filtering, frequency multiplication and frequency division; (2) arrangements known as indirect frequency synthesizers wherein a voltage controlled oscillator is phase-locked to a reference signal to thereby supply an output signal at a frequency that is a rational mathematical function of the reference signal frequency; and, (3) digital synthesizer arrangements wherein the frequency of a reference signal determines an invariant sampling interval and real-time digital computation is employed to calculate a sequence of signal amplitudes which are filtered to supply an analog signal of the desired frequency. Although each type of frequency synthesizer exhibits advantages and disadvantages that can make a particular type of synthesizer preferable in a specific design situation and one or more of these synthesis techniques can be combined in a single sytem, the relatively low parts count and ability of phase-locked loop synthesizers to operate at frequencies up to several hundred megahertz with a one octave or wider bandwith has led to widespread usage of phase-locked loop arrangements.
With respect to active oscillator circuits, many types of resonant feedback structure such as inductance-capacitance networks, crystals, and distributed parameter networks (e.g., resonant cavities) are utilized in the art, with the particular resonant structure employed in each circuit design being selected on the basis of the signal frequencies to be generated as well as other desired performance characteristics. In this regard, and wih respect to relatively wideband signal sources for operation over the portion of the frequency spectrum extending between the upper portion of the VHF range and the middle portion of the UHF range (e.g., between one and several hundred megahertz), tunable resonant cavities or resonant networks employing inductors and variable capacitors of conventional construction or of the semiconductor variety (e.g., varactor diodes) are commonly employed.
It is well-known that, regardless of type of circuitry employed, no physically realizable signal source provides an output signal that is a perfect sinusoid. In particular, variations in phase or frequency and variations in amplitude are inherently associated with all signal sources because of random internal circuit noise and because of coherent noise such as that generated due to nonlinear circuit operation and bias supply ripple. Since the amplitude of the signal supplied by conventional signal sources such as active oscillator circuits and frequency synthesizer arrangements can be made relatively invariant by utilization of conventional design techniques, the primary noise component of state-of-the-art signal sources is generally phase noise.
Although the phase noise of a signal source can be specified in a number of different manners, the usual procedure is to specify the ratio between the signal power at the desired signal frequency (carrier frequency) to the power within a one hertz bandwidth that is offset from the carrier frequency by a specific amount to provide a signal-to-noise ratio (S/N) in decibels/hertz. With respect to active oscillator arrangements, the S/N is directly related to the Q of the resonant circuit and the maximum power available to the active device and is inversely related to the noise figure of the active device. Thus, it can be recognized that active oscillators utilizing high Q tunable resonant cavities can be designed to exhibit a much higher signal-to-noise ratio than active oscillators which use other types of resonant structure (e.g., L-C circuits utilizing variable inductors and/or capacitors). Accordingly, prior to this invention, active oscillators including tunable resonant cavities have generally been utilized in design applications calling for the generation of a variable frequency signal that exhibits very low phase noise.
Although signal sources employing tunable resonant cavities are capable of meeting very demanding phase noise requirements, various disadvantages and drawbacks are encountered. For example, resonant cavities that are dimensioned for operation at frequencies below the microwave range are rather large and oftentimes the signal source must generate a signal at a high frequency and utilize down conversion techniques to produce the desired signal frequency. Such a requirement not only reduces the signal-to-noise ratio that could otherwise be achieved, but increases the complexity and cost of the overall signal source. Further, and of considerable importance in many design situations, such resonant cavities must be mechanically tuned and thus must be manually adjusted each time the signal frequency is changed. Since precise and rapid control of signal frequency is often required, such resonant cavity controlled active oscillators have presented a cumbersome and time-consuming procedure. This disadvantage of the prior art resonant cavity oscillator circuit is especially important in situations in which it is desired or necessary to remotely and rapidly adjust or program the output frequency of a signal source by means of an electrical signal.
Although phase-locked loop frequency synthesizers can be electronically controlled and thus provide remote frequency programming, such circuits have not been suitable for use in situations calling for extremely low phase noise. First, as previously mentioned, phase-locked loop systems include a voltage controlled oscillator--which is itself an active oscillator network. In this regard, a conventional phase-locked loop does not provide significant improvement over performance of the VCO alone since phase perturbations at frequencies outside the phase-locked loop bandwidth are attenuated only by circuit structure that lies between the VCO output terminal and frequency control terminal. For example, a loop filter and/or a filter for suppressing the reference signal frequency are often included in conventional phase-locked loop systems and may partially suppress system noise that lies outside the loop bandwidth. However, since the frequency response of these filter networks, as well as the loop bandwidth, is determined by various design tradeoffs that must be made to achieve desired values of basic system performance characteristics such as the time required to switch the system to a new frequency, these networks cannot substantially improve phase noise performance without substantial sacrifice in system performance. Accordingly, although judicious design of phase-locked loop systems utilizing presently available components can result in noise performance as low as 100 to 120 db/hertz at an offset of 20 kilohertz, voltage controlled oscillators have not been available which permit the artisan to utilize phase-locked loop synthesizers in design situations requiring extremely low phase noise.
Accordingly, it is an object of this invention to provide a signal source which supplies an electrical signal at a selectable signal frequency wherein the signal exhibits very low phase noise.
It is another object of this invention to provide such a signal source wherein the frequency of the output signal is selected in response to an electrical signal.
It is another object of this invention to provide a low phase noise signal source capable of utilization within a phase-locked loop frequency synthesizer to thereby provide a low noise signal source system exhibiting ease of frequency selection and substantial frequency stability while simultaneously achieving low noise operation over a wide range of operating frequencies.
Still further it is an object of this invention to provide a low noise signal source suitable for use in a phase-locked loop synthesizer wherein the noise performance of the phase-locked loop is greatly improved without deleteriously affecting other performance characteristics of the phase-locked loop.